12 SOUTHERN CALIFORNIA GEOLOGY 



The pressures or stresses which produce the strains 

 are those incurred by the processes of isostasy, as else- 

 where herein described. The sudden releases of strain 

 result from pressure which takes place along the lines 

 of weakness, usually fault lines as above stated. The 

 causes of these movements are not definitely known. 

 They are discussed in another chapter. 



Inasmuch as an earthquake is ordinarily the result of 

 a displacement along a limited portion of a fault line, 

 a faut line of magnitude, like the master faults of 

 Southern California, represents the site and results 

 of long series of small earth movements which have 

 taken place successively during extensive periods of 

 past time. 



There are other kinds of earthquakes, such as ob- 

 viously accompany volcanism, that are of secondary 

 interest and need not be described at present. Many 

 students and masters of geology have given great 

 thought to the study of earthquakes and the problems 

 which they present. There is a vast accumulation of 

 literature upon the subject and several scientific socie- 

 ties are devoted to it. 



METHODS OF STUDY 



Every aspect of earthquakes is being studied at 

 present, including their relations to various phases of 

 nature, such as tides, the earth's rotation, seasons of 

 the year, climate, sunshine, rain and barometric pres- 

 sure, but the results are foggy, and we still have much 

 to learn. 



Seismologic laboratories with elaborate apparatus 

 have been located in various parts of the world. Much 

 has been ascertained concerning earthquakes, but we 



